Conventionally, as a high-frequency filter circuit, there is the high-frequency filter circuit managed as a distributed element circuit shown in FIG. 16. As shown in FIG. 16, this high-frequency filter circuit employs a microstrip line as a high-frequency transmission line that forms the basis of a distributed element. In FIG. 16, there are shown a dielectric substrate 85 of ceramic or the like, a GND pattern 86 on the lower surface of the dielectric substrate 85, an input line 81 which is provided on the dielectric substrate 85 and whose one end serves as an input port, an output line 82 which is provided on the dielectric substrate 85 and whose one end serves as an output port, and λ/2 resonators 84a and 84b. The λ/2 resonators 84a and 84b are microstrip lines whose length is designed roughly to λ/2 with respect to the wavelength λ of the center frequency of the filter, and their both ends are open ends. There are also shown a gap 87a provided between the input line 81 whose one end is the input port and the λ/2 resonator 84a, a gap 87b provided between the output line 82 whose one end is the output port and the λ/2 resonator 84b, and a gap 88 provided between the λ/2 resonator 84a and the λ/2 resonator 84b. 
The above kind of high-frequency filter circuit, which is able to form a filter circuit of printed wiring of only one layer and excellent in terms of productivity and cost, has often been used in the frequency band of about 5 to 30 GHz. Furthermore, in recent years, the filter circuit has been used also in the millimeterwave band of 30 to 60 GHz.
FIG. 17 shows the equivalent circuit of the high-frequency filter circuit shown in FIG. 16. In FIG. 17, there are shown capacitors C51, C52 and C53 and an inductor L51. It is generally known that the filter characteristic of this high-frequency filter circuit becomes as shown in FIGS. 18 and 19. In FIGS. 18 and 19, the reference numeral S11 denotes a parameter that represents a reflection coefficient, and the reference numeral S21 denotes a parameter that represents a transmission coefficient. FIG. 18 is a graph of a wide band, and FIG. 19 is a graph in which the portion including the passband and the attenuation band is enlarged. In this case, the capacitor C51 is 0.03661 pF, the capacitor C52 is 0.05270 pF, the capacitor C53 is 0.02884 pF, and the inductors L51 and L52 are each 0.01699 nH. In FIG. 19, the passband and the attenuation band (unnecessary wave band) of the required filter specifications are indicated by hatched lines in the case where there is supposed an application of removing the unnecessary wave band (image band attributed to the mixer) set at 55 to 57 GHz with the passband of the filter set at 60 to 62 GHz for the sake of comparison with the graph of the high-frequency filter circuit of this invention described later.
Although several graphs are hereinafter inserted in the present specification, the three kinds of graphs of simulation results by an equivalent circuit of a lumped element, simulation results by a distributed element equivalent circuit and measurement results of actually conducted experiments exist in mixture inevitably for convenience in explanation. Among others, the simulation results centered on a lumped element as in FIGS. 18, 19 and 10 are to clarify the principle of operation of a circuit, and no consideration is provided for loss in the circuit elements. Therefore, insertion loss is calculated underestimated. In the prior art high-frequency filter circuit of FIG. 16, even a minimum insertion loss in the microwave band, in which the frequency is comparatively low, generally amounts to about 2 to 3 dB though it is changed depending on the bandwidth and attenuation.
The aforementioned high-frequency filter circuit has a first problem that steepness in the filter characteristic is low particularly when used in a superhigh frequency band like the millimeterwave band. In general, the most striking feature in the specifications required for the filter circuit of the millimeterwave band resides in its steepness. For example, reference is made to a radio communication device of 60-GHz band taken as an example. Even in the radio communication device of 60-GHz band, signal processing in the IF circuit is usually performed in a low frequency band of about 1 to 2 GHz. Subsequently, by being mixed with a local signal of, for example, 59 GHz, the signal is finally upconverted to the millimeterwave band of 60 to 61 GHz. Here is considered the filter specifications required when image rejection is effected by a millimeterwave band filter in such a radio communication device. When the signal is upconverted to the millimeterwave band of 60 to 61 GHz, the image band is located in the position of 57 to 58 GHz. That is, the 58-GHz band located only 2 GHz away from the passband of 60 GHz becomes the inhibition zone, and this means that the frequency interval has a separation of only about 2÷60=3.3% in terms of a ratio with respect to the band. Accordingly, there is required extremely high steepness such that the signal is attenuated by a minimum of about 15 dB within the interval of the ratio of only about 3% with respect to the band as a filter specification.
However, in the case of the prior art high-frequency filter circuit of FIG. 16, there can be achieved only the bandpass characteristic of gentle slopes as is apparent from the graphs of FIGS. 18 and 19, and high steepness cannot be obtained. In the case of such a filter circuit, there is known the method of increasing the number of stages of the circuit, i.e., increasing the number of λ/2 resonators in order to increase the steepness. However, the above method practically has a slight improvement of steepness in the vicinity of the passband, and there is an adverse effect that the following second and third problems described below disadvantageously increase, lacking practicability.
As the second problem of the aforementioned high-frequency filter circuit, there is a problem that insertion loss is large. It is generally known that the parasitic loss of the circuit sharply increases in the superhigh frequency band like the millimeterwave band. Particularly, in the case of the high-frequency filter circuit shown in FIG. 16, it can easily be estimated that this parasitic loss becomes significant according to its structure. An electric signal entering from the input port of the input line 81 appears at the output port of the output line 82 after it passes all the way through a long path extended from the gap 87a, the λ/2 resonator 84a, the gap 88, the λ/2 resonator 84b and the gap 87b. Through this path, a conductor loss, a radiation loss and a dielectric loss are generated and added up in the gaps 87a, 88 and 87b and the resonators 84a and 84b. That is, the structure in which the series signal path is extremely long itself has the cause of the unavoidable loss increase.
Further, as the third problem of the high-frequency filter circuit, there is the problem that the circuit area is large. In the superhigh frequency band like the millimeterwave band, it is known that reducing the parts count and the number of connection portions between circuits by integrating a plurality of circuits into one chip on an MMIC (Monolithic Microwave Integrated Circuit) is a very effective method in terms of both an improvement in the electrical performance and an improvement in the manufacturing cost. The same thing can be said for the high-frequency filter circuit, and there is a great need for integrating the filter circuit with the amplifier circuit and the mixer circuit connected before and behind the circuit into one chip on an MMIC. On the other hand, it is required to reduce the circuit area in order to reduce the chip cost of the MMIC. In the case of the high-frequency filter circuit shown in FIG. 16, there is a drawback that the dimension particularly in the direction A in FIG. 16 disadvantageously becomes extremely long since it has the structure in which the input line 81, the λ/2 resonator 84a, the λ/2 resonator 84b and the output line 82 are connected in series. Even in the millimeterwave band in which the wavelength λ is short, it is normal that the λ/2 dimension is about 1 mm in, for example, the 60-GHz band. From the viewpoint of the dimensions of a millimeterwave-band MMIC that often has in general a size of about 1 to 2 mm square, the size A of FIG. 16 disadvantageously becomes an unacceptable large size.
Moreover, as a prior art high-frequency. communication device, there is the millimeterwave band communication device shown in FIG. 20. As shown in FIG. 20, this millimeterwave band communication device is provided with two mixers 91 and 92 into which a TV signal is inputted, a local oscillator 93 that supplies a local signal to the mixers 91 and 92, an amplifier (hereinafter referred to as an amp) 94 that amplifies the output signals outputted from the two mixers 91 and 92 and an antenna 95 to which the output of the amp 94 is connected. The two mixers 91 and 92 constitute a balance type mixer 90.
For the purpose of image rejection in the millimeterwave band communication device shown in FIG. 20, there has often been used not a filter but a balance type image rejection mixer. This is because it has been difficult to obtain a filter of excellent steepness in the millimeterwave band. However, the balance type image rejection mixer generally has a drawback that the bandwidth is narrow, and it has been difficult to meet the requirements of the TV signal transmission system that has a bandwidth of up to 2 to 3 GHz by only the balance type image rejection mixer (refer to the reference document of K. Hamaguchi et al., “A Wireless Video Home-Link Using 60 GHz Band: A Concept of Developed System”, Proc. of EuMC, vol.1, pp.293–296, 2000”). Moreover, in the case where the balance type image rejection mixer is employed, it is usual that the chip area is increased double or more in comparison with that of the ordinary mixer circuit that is not the balance type. Accordingly, there are the problems of an increase in the chip unit cost and difficulties in attempting to integrate other circuits (amp circuit and so on) on an identical chip any further.
Furthermore, as another prior art high-frequency communication device, there is the millimeterwave band communication device shown in FIG. 21. As shown in FIG. 21, this millimeterwave band communication device is provided with a mixer 101 into which a TV signal is inputted, a local oscillator 102 that supplies a local signal to the mixer 101, a filter 103 that effects image rejection of the signal outputted from the mixer 101, an amp 104 that amplifies the signal outputted from the filter 103 and an antenna 105 to which the output of the amp 104 is connected.
For the purpose of image rejection in the millimeterwave band communication device shown in FIG. 21, it has often been the case where a waveguide filter has been used as the filter 103 capable of obtaining high performance also in the millimeterwave band. However, in this case, there have been the drawbacks of difficulties in electrical connection between the waveguide and the MMIC as well as the expensiveness, large size and heavy weight of the waveguide filter itself.